This application claims the benefit of Application No. 00307301.2 filed on Aug. 24, 2000 under 35 U.S.C. xc2xa7119, xc2xa7365(a), or xc2xa7365(b). (For originals)
1. Field of the Invention
The present invention relates to detecting the presence of a crack in an object of electrically conducting material by means of eddy currents induced in the object. The object can be a plate, such as a support plate or a shell, such as a wall. The electrically conducting material can be carbon steel or stainless steel. More in particular, the present invention relates to detecting cracks in an object of electrically conducting material that is covered by a relatively thick layer of a non-conductive material.
2. Description of Related Art
International patent application publication No. 95/00 840 discloses a method of detecting cracks in an object of electrically conducting material. The known method comprises inducing an eddy current into a portion of the object with an abruptly changing magnetic field; while the induced eddy current decays in the object portion, detecting the decay of the induced eddy current; determining with respect to time the derivative of the decay of the induced eddy current; determining a value from the derivative which is representative of the thickness of the portion; determining, by use of a magnetic flux leakage method, the wall thickness of the object portion; and inferring that a plurality of cracks is present where a reduction in wall thickness is indicated by the derivative of the decay and no reduction in wall thickness is indicated with the magnetic flux method.
Thus in the known method, two different methods are required to determine the presence of a crack: an eddy-current method and a magnetic flux leakage method. However, the magnetic flux leakage method can only be applied to relatively small lift-offs, in practice only up to 10 mm. Because the steel needs to be magnetized, the magnetic flux leakage method becomes impractical and insensitive for larger lift-offs. Furthermore, a magnetic flux leakage method requires a bulky inspection apparatus that consumes much electrical power. This is a drawback for the inspection of bridges, since the inspection tools need to be portable and battery operated.
It is an object of the present invention to provide a method of detecting cracks in an object of electrically conducting material wherein only one inspection method is used.
To this end the present invention provides a method of detecting cracks in object of an electrically conducting material that is covered by a non-conductive layer using a probe comprising a transmitter system for inducing eddy currents in the object, and a receiver system for providing a signal indicative of the strength of a magnetic field or changes in the strength of a magnetic field, which method comprises:
a) selecting a set of points of the object which are to be inspected;
b) selecting a first inspection point from the set;
c) positioning the probe at the selected inspection point, activating the transmitter system to induce eddy currents in the object;
d) creating a record of the signal provided by the receiver system over a period of time;
e) examining the record and comparing it to reference records, wherein the presence or absence of a crack can be inferred from the comparison; and
f) selecting a next point of the set and repeating steps c) through e) until all points have had their turn.
In the specification and in the claims, the term xe2x80x98non-conductive layerxe2x80x99 is used to refer to a layer of material that has a conductivity that is much lower than the conductivity of the object.